In a medium bowl, whisk together the gelatin and 3 tablespoons cold water.
Put the Nutella and salt in a medium bowl and set aside.
In a medium saucepan, bring the cream and vanilla to a boil. Pour 1/3 of the hot cream mixture over the gelatin and whisk it well. Then pour the gelatin mixture back into the remaining cream.
Pour about 1/3 of the cream mixture over the Nutella. Whisk well to form a smooth paste. Add the remaining cream mixture and whisk well to combine. Whisk in the milk.
Strain the mixture through a fine sieve into a bowl, then divide it among eight ramekins, glasses or cups. Refrigerate until set, about 3 hours – the panna cotta can be kept, loosely covered with plastic wrap, in the refrigerator for up to 2 days.
They can be served in the ramekins or dipped in hot water and unmolded onto a serving plate.